The NKG2D receptor triggers natural killer cells and costimulates T cells upon engagement of ligands that are absent from most normal cells but can be induced by cell stress. Ligands of NKG2D in humans include the major histocompatibility complex class I-related chain A (MICA), which is frequently expressed in epithelial tumors and certain autoimmune disease lesions. Binding of MICA to NKG2D may thus promote tumor immunity and worsen autoimmune disease progression. However, we have uncovered negative immune regulation whereby persistent NKG2D costimulation as a result of chronic MICA expression causes proliferative expansions of normally rare NKG2D+CD4+ T cells that produce immunosuppressive cytokines. These T cells, which represent a distinct T cell subset, recognize normal tissue self antigens and display pronounced T cell antigen receptor (TCR) repertoire contractions. Upon NKG2D costimulation, they also produce death receptor Fas ligand but are themselves protected from Fas-induced apoptosis or growth arrest, as opposed to other T cells that do not receive survival signals. The objective of this application is to advance fundamental knowledge of these previously unrecognized suppressive NKG2D+CD4+ T cells and their regulation by NKG2D, and to explore their significance in a human autoimmune disease. We propose that the role of the NKG2D+CD4+ T cells is reminiscent of classical regulatory T cell activities, with similarly opposed negative and protective effects in tumor immunity and autoimmune diseases, respectively. This idea is supported by increased NKG2D+CD4+ T cell frequencies in late-stage cancer patients with failed tumor immunity. Conversely, we have compelling evidence for a role of these T cells in attenuating autoimmune disease severity, as assessed in systemic lupus erythematosus (SLE). The studies proposed in this application will shed light on newly discovered functions in human homeostatic immune regulation and provide vital information for probing the utility of NKG2D+CD4+ T cells as a modality for treatment of autoimmune disease. Specific Aim 1. Determine the functional attributes and significance of NKG2D+CD4+ T cells as regulators of disease activity in pediatric and adult SLE. Specific Aim 2. Define how NKG2D signaling activates anti-apoptotic pathways and thereby promotes NKG2D+CD4+ T cells survival. Specific Aim 3. Identify dominant self antigens that are recognized by NKG2D+CD4+ T cells in SLE to elucidate their cellular origin and tissue specificity and enable MHC tetramer-based T cell tracking. PUBLIC HEALTH RELEVANCE: The proposed research addresses a newly discovered regulatory mechanism whereby immune system responses are attenuated in cancer and autoimmune disease patients. Better knowledge of this mechanism may allow inhibition of this mechanism in cancer patients, thus helping the immune system to attack cancer. Conversely, enhancing this mechanism in autoimmune disease patients may lower disease activity. The results of our studies may thus lead to development of new therapies.